1. Field of the Invention
The present invention relates to a method of manufacturing continuous fiber-reinforced thermoplastic (hereinafter abbreviated to "FRTP") prepregs and an apparatus for carrying out the same.
2. Description of the Prior Art
The continuous FRTP prepreg is a thermoplastic forming material reinforced by metallic fibers, vegetable fibers, organic fibers or inorganic fibers such as carbon fibers and glass fibers. As is well known, the continuous FRTP prepreg is formed in pellets, ribbons for filament winding, UD sheets or cloth sheets, and these materials are processed through an injection molding process, a press forming process, a bag forming process or a filament winding process to manufacture structural members and machine parts.
Various methods of manufacturing continuous FRTP prepregs have been proposed. Japanese Patent Publication No. 52-3985 discloses a method, in which a fiber bundle, namely, a bundle of filaments such as a tow or a bundle of staple fibers, is impregnated with a resin powder, and then the fiber bundle impregnated with the resin powder is heated to melt the resin powder. Japanese Patent Laid-open No. 60-36136 discloses a method, in which the surface of a fiber bundle is coated with a molten thermoplastic resin, and then the molten thermoplastic resin is made to permeate the fiber bundle by belt press. Japanese Patent Laid-open No. 58-211415 discloses a method, in which a sandwich of resin sheets and fiber sheets is combined by heating. Japanese Patent Publication Nos. 60-6764 and 61-4629 disclose methods, in which a fiber bundle is impregnated with a resin emulsion, the fiber bundle impregnated with the resin emulsion is dried, and then the fiber bundle is coated with the resin in the crosshead of an extruder.
FIG. 10 illustrates an exemplary impregnating device employed in a conventional apparatus for manufacturing a continuous FRTP prepreg. A resin solution 6 prepared by dissolving a resin in an appropriate solvent is contained in an impregnating tank 4. A filament bundle 1 is fed by feed rollers 2 and is guided by guide rollers 3 and 5 so as to pass through the resin solution 6 to impregnate the filament bundle 1 with the resin solution 6. Then, the filament bundle 1 impregnated with the resin solution 6 is subjected to a solvent extraction process to remove the solvent so that only the viscous resin remains in the filament bundle 1. Thus a continuous FRTP prepreg having high formability. This conventional apparatus, however, has a problem that it is difficult to remove the solvent completely from the filament bundle and the residual solvent deteriorates the characteristics of the continuous FRTP prepreg and those of formed products manufactured by using the continuous FRTP prepreg.
To avoid such adverse influence of the solvent on the products, methods of manufacturing a continuous FRTP prepreg without using the solvent, such as a hot-melt method (pultrusion method) and a fluidized bed method, have been proposed.
FIG. 11 illustrates part of an impregnating device employed in carrying out the previously proposed hot-melt method. A molten resin 12 is contained in a heating tank 10 provided with a heater for heating the molten resin 12, and an orifice 13 formed in the bottom wall thereof. A filament bundle 1 introduced through the upper opening of the heating tank 10 into the heating tank 10 is passed through the molten resin 12 and is pulled out through the orifice in a prepreg. Then, the prepreg is shaped by banding rollers 8a and 8b, and then the shaped prepreg is heated again by a heating roller 9 so that the resin is sufficiently fluidized to permeate the filament bundle 1 perfectly.
In the conventional fluidized bed method, a fiber sheet, such as a cloth sheet or a UD sheet, is immersed in a molten resin contained in a heating tank as the fiber sheet is passed through the heating tank to manufacture a sheet-form continuous FRTP prepreg through a single process.
Both the conventional hot-melt method and the fluidized bed method have a problem that it is very difficult to maintain uniform distribution of the temperature of the molten resin in the heating tank. The temperature of the molten resin is not distributed uniformly in the heating tank due to the low thermal conductivity of the resin, and thereby the viscosity of the molten resin varies from place to place in the heating tank. Consequently, the filament bundle or the like is impregnated partially insufficiently with the molten resin and, in many cases, voids are formed in the resin-impregnated filament bundle. To impregnate the fiber bundle forcibly with the molten resin and to remove bubbles remaining in the fiber bundle, the fiber bundle is hot-pressed by heating means, such as the heating roller 9, after being immersed in the molten resin. However, the conventional method is unable to remove the bubbles perfectly from the resin-impregnated fiber bundle, which deteriorates the characteristics of products formed by using the resin-impregnated fiber bundle.
Furthermore, portions of the fiber bundle insufficiently impregnated with the molten resin due to viscosity variation in the heating tank entails irregular strength distribution in the prepreg and the large frictional resistance of the molten resin heated at a low temperature damages the fiber bundle, so that weak portions are formed in the prepreg. Since the thermoplastic resin is heated to a high temperature near the critical temperature at which the thermoplastic resin is decomposed and the thermoplastic resin permeated the fiber bundle is heated again by the heating roller 9, a large quantity of energy is wasted. Such disadvantages become further conspicuous when a heat-resistant thermoplastic resin having a high melting point, such as polyetheretherketone or polyestersulfone, is used.